In a direct injection internal combustion engine, a fuel injector is provided to deliver a charge of fuel to a combustion chamber prior to ignition. Typically, the fuel injector is mounted in a cylinder head with respect to the combustion chamber such that its tip protrudes slightly into the chamber in order to deliver a charge of fuel into the chamber.
One type of fuel injector that is particularly suited for use in a direct injection engine is a so-called piezoelectric injector. Such an injector allows precise control of the timing and total delivery volume of a fuel injection event. This permits improved control over the combustion process which is beneficial in terms of exhaust emissions.
A known piezoelectric injector 2 and its associated control system 4 are shown schematically in FIG. 1. The piezoelectric injector 2 is controlled by an injector control unit 6 (ICU) that forms an integral part of an engine control unit 8 (ECU). The ECU 8 monitors a plurality of engine parameters 10 and calculates an engine power requirement signal (not shown) which is input to the ICU 6. In turn, the ICU 6 calculates a required injection event sequence to provide the required power for the engine and operates an injector drive circuit 12 accordingly. The injector drive circuit 12 is connected to the injector 2 by way of first and second power supply leads 14, 16 and is operable to apply a differential voltage to the injector 2, via the leads 14, 16.
The piezoelectric injector 2 includes a piezoelectric actuator 18 that is operable to control the position of an injector valve needle 20 relative to a valve needle seat 22. The piezoelectric actuator 18 includes a stack 24 of piezoelectric elements that expands and contracts in dependence on the differential voltage supplied by the drive circuit 12. The axial position, or ‘lift’, of the valve needle 20 is controlled by varying the differential voltage across the actuator 18.
By application of an appropriate voltage differential across the actuator 18, the valve needle 20 is either caused to disengage the valve seat 22, in which case fuel is delivered into an associated combustion chamber (not shown) through a set of nozzle outlets 26, or is caused to engage the valve seat 22, in which case fuel delivery through the outlets 26 is prevented.
A piezoelectrically controlled injector of the aforementioned type is described in the Applicant's European Patent Numbers EP 1174615 B and EP 0995901 B. The injector 2 is of the deenergise-to-inject type in which a reduction in the voltage across the actuator 18 initiates an injection event. As an injector spends the majority of its life in a non-injecting state, in injectors of the deenergise-to-inject type the voltage across the actuator 18 is relatively high for the majority of its life (at least 90%). It has been identified that this may have a detrimental effect on injector service life as a correlation exists between the life of the piezoelectric actuator and the amount of time for which the actuator has a relatively high voltage across it.
It is an object of the present invention to provide an improved method of operating a piezoelectrically actuated fuel injector which alleviates the aforementioned problem.